Turn up the heat? Just kidding. I personally wouldn't rely on a medication - it's all pretty habit-forming and can have some unpleasant side effects. The only thing that's ever worked for me is making sure I knew my stuff thoroughly, by really studying what I have to present and doing it in such a way that I could talk about my subject to someone without having the paper there with me. I find it's not the presentation itself that worries people, it's the thought that your brain suddenly dries up and you don't know what else to say. If you learn about your material like it's something you're genuinely interested in, all you'll be doing is talking about it instead of presenting. That's a hell of a difference, at least to me.